The treatment of biological sludges, including municipal sewage sludges, has become an increasing large problem due, in large part, to the increasing volumes of such wastes and the increased environmental awareness and regulations covering the processing and disposal of such wastes. Current and past procedures for the treatment and disposal of municipal sewage sludges can generally be divided into four major processes: lime stabilization, composting, application of raw sewage sludge to agriculture land, and incorporation of raw sewage sludge in landfill sites. Generally, the application of raw sewage to agricultural land or disposal in landfills is no longer a viable or responsible option in the industrialized world. Any such disposal by these just-mentioned practices is likely be even less utilized in the future due to increasingly strict environmental regulations and the shortage of land for landfill purposes. The other two procedures--lime stabilization and composting--continued to be used in spite of significant drawbacks and problems associated with the processes.
In the composting method, municipal sludge is mixed with aeration type materials, such as wood chips or chopped tires, and placed directly on the ground to decompose and dry. The material is turned several times in order to further aerate the pile. At some later time --often on the order of months--the material becomes dried sludge. During the composting period, this method can often result in significant complaints from neighbor due to the sewage-like smell and the attraction of disease vectors such as rodents, flies, and the like. Problems associated with noxious odor can become especially troublesome if the composting piles are not carefully monitored and turned at the appropriate times to insure adequate aeration during decomposition. In addition, such composting procedures require significant land resources to compost the sewage sludge from even small municipal districts.
Lime stabilization has been used to disinfect and deodorize sewage wastes since at least the time of the Roman Empire. Generally lime stabilization employs lime, especially high calcium carbonate lime, mixed at a rate of about 10 weight percent or higher with the sewage sludge. The mixture is then stored, often directly on the ground, for later application to agriculture lands. The materials produced vary considerably in nutrient value, pathogen content, consistency, and suitability for land application. The lime stabilization process can also produce significant amounts of a leachate containing high levels of nitrates and heavy metals. Normally this leachate is either allowed to drain directly into the ground (and, perhaps, utimately into ground or surface water) or is simply collected and piped back into the municipal sewage treatment system to be passed through the system again. Neither method of disposing this leachate is desirable.
Both of these processes, as well as subsequent disposal or utilization of the resulting treated sludge, have, as noted above, significant drawbacks, limitations, and problems. Handling and subsequent application of treated sludge prepared from either composting or lime stabilization procedures are especially troublesome. For example, the bulk treated sludge must be transported to the application site where it is usually dumped directly on the ground in a large pile or piles. Any leachate draining from these piles will directly contaminate the ground and, in some cases, the ground or surface water. From such piles, the material must be loaded into spreading equipment and then applied over the target land. In addition to packing and compacting the land, this procedure is both slow and cumbersome. The amount of material to be applied over a given land area is determined more by the amount of treated sludge to be disposed and the area of land available rather than the nutrient needs of the land. In addition, because the texture of the materials vary considerably depending on the temperature and age of the treated sludge, uniform application is difficult and depends almost entirely upon the skill and experience of the applicator. In sum, application of such materials is destructive of the land, unpleasant and perhaps unhealthy due to dust and noxious odors associated with the treated sludge, and provides a low-grade fertilizer which cannot easily be applied in a uniform and consistent manner. Due to these problems, the agriculture market demand for treated sludge is essentially nonexistent. Rather, the contractor who generates the treated sludge must locate the land for application and then pay for freight, equipment, and application costs. Such costs, of course, add directly to the costs municipalities must bear to treat and dispose of their wastes.
Recently the United States Environmental Protection Agency (EPA) has promulgated rules governing the treatment of municipal sludges and the application or incorporation of such treated materials onto or into the ground. Under Title 40 of the Code of Federal Regulations, Part 257 (i.e., 40 CFR Part 257), a Process to Significantly Reduce Pathogens (PSRP) must be used where sewage sludges are applied to land surfaces or incorporated into soil where the public has access to the land within twelve months of the application or incorporation or where grazing animals whose products are consumed by humans are allowed to graze the land within one month of the application or incorporation. Under these same regulations, a Process to Further Reduce Pathogens (PFRP) must be used where sewage sludges are applied to land surfaces or incorporated into soil where crops for direct human consumption will be grown within the next eighteen months. Currently approved PSRP processes include aerobic digestion, air drying, anaerobic digestion, composting, and lime stabilization. Provision is also made for other treatment methods which provide equivalent reduction in pathogens and vector attraction relative to the just listed methods. Currently approved PFRP processes include composting at elevated temperatures, heat drying, heat treatment, and thermophilic aerobic digestion. Other methods, if shown to provide equivalent reduction in pathogens and vector attraction to the listed PFRP methods, can be used.
In U.S. Pat. No. 4,554,002, a method for treating wastewater treatment sludge was described. This method involved the addition of kiln dust from either cement or lime plants to the sludge to reduce pathogens and dry the waste prior to land application. The materials were exposed to ambient temperatures without compacting for a time sufficient to produce a so-called "disintegratable, friable product which can be applied to land as a soil conditioner." According to this patent, the kiln dust and waste sludge mixture must be cured "at ambient temperatures at least three days or preferably until it achieves sufficient cohesiveness to be granulated, shredded, crushed, etc., into particles resembling a bank run sand and gravel mixture."
In U.S. Pat. Nos. 4,781,842 and 4,902,431, a method is disclosed for decontaminating wastewater sludges wherein lime or kiln dust are mixed with the wastewater sludge in sufficient quantity to raise the pH of the mixture to 12 for at least two hours and then drying the resulting mixture with an aeration process. In order to achieve PFRP reduction in pathogens it was necessary to dry the lime stabilized sludge with aeration drying such that the sludge contains less than 20 percent, and preferably less than 10 percent, moisture content. The aeration drying process consisted of applying the lime-treated sludge to the ground in windrows that were mixed two to three times a week with a "Brown Bear" aerating device. After considerably drying times--often 30 or more days, often considerably more, depending on the ambient temperature and precipitation conditions--a material was obtained that could be applied to the land as fertilizer or as a landfill cover.
The lime stabilization procedures of U.S. Pat. Nos. 4,554,002, 4,781,842, and 4,902,431 have several drawbacks. In each case, the materials produced are not consistently in the form of granules or pellets which can be easily applied to the land using conventional agricultural dispersing methods. The drying procedures take considerable time and are very dependent on weather conditions. It may be merely fortuitous if the treated sludge is sufficiently dry and, therefore, ready for land application at approximately the same time the land is available for application; thus, in many instances, the only reliable source of land for applying such treated sludge may be land that is held out of production during a given growing season. The drying procedures also require significant land areas which must essentially be dedicated to such operations. Suitable land for drying the lime-treated sludge (i.e., relatively low cost land with low potential for migration to ground or surface water), if available at all, may be significantly removed from sewage treatment and collection facilities and from the agricultural land to which the dried treated sludge is to be applied, thereby increasing dramatically the transportation and overall costs of the treatment process. Applying the materials directly on the ground during the drying process allows any leachate formed to drain directly into the ground and, perhaps, eventually into the ground or surface water. Such leachate may contain significant levels of nitrates and heavy metals. To Applicant's knowledge, the procedures described in these three patents have not been used or adopted on a large scale. Although demonstration trials have been run using municipal sludge from several municipalities, these procedures have not been adopted in these cities.
It is desirable, therefore, to provide a method of treating biological sludge, including municipal sewage sludge, whereby a granulated or pelletized product suitable for use as an organic base fertilizer can be obtained in a relatively short time in a reproducible and consistent manner. It is also desirable to provide a method of treating biological sludge, including municipal sewage sludge to obtain a granulated or pelletized fertilizer-type product which is physically similar to commercially available fertilizer products and which can be applied using commercially available application procedures and equipment. It is also desirable to provide a granulated or pelletized fertilizer product from municipal sewage sludge which has both plant nutrient value and economic value. It is also desirable to provide a granulated or pelletized fertilizer product from municipal sewage sludge which does not significantly deteriorate over reasonable periods of time and can, therefore, be applied to agricultural land at the optimum time in the agricultural cycle. It is also desirable to provide a method of treating municipal sewage sludge whereby reduction of the pathogen levels and the vector attractiveness of the resulting materials is sufficient to meet or exceed current EPA guidelines. It is also desirable to provide a method of treating municipal sewage sludge whereby leachate from the municipal sewage sludge is contained within the process and is not allowed to escape back into the municipal system or directly into the ground as well as the ground or surface water. The current method and the resulting organic base fertilizer produced from that method, as described herein, are thought to meet these criteria.